A multi-band mobile station, such as a radiotelephone or personal communicator, is capable of operating in more than one system, each having different frequency bands. By example, and for a dual band mobile station, the mobile station may be capable of operating in a digital cellular mode (e.g., DCS1900 at about 1900 MHz) and an analog cellular mode (e.g., IS-41 (AMPS) at about 830 MHz).
An important component of a mobile station is a transmitter power control subsystem. In general, the transmitter power control subsystem operates by directly sensing the. transmitted RF power, usually with an RF directional coupler and a detector diode, and then using the detected signal to close a power control loop by comparing the detected RF power with a value of a transmitter power control signal (TXC).
By example, and referring to FIG. 3, a conventional arrangement for a transmitter power control subsystem of a mobile station that operates in two bands (i.e., Band1 and Band2) includes, for each band, a variable gain amplifier (VGAn), a power amplifier (PAn), and a directional coupler (DCn) coupled to an antenna element (ANTn). Connected to an output of the directional coupler is a detection diode (Dn) having an output connected to an input of an error amplifier (An). A second input of the error amplifier is connected to the transmit power control signal (TXCn). An error signal, representing the difference between the transmitted power commanded by the TXC signal and the actual RF power measured by the directional coupler, is connected to the VGA for either increasing or decreasing the gain of the amplifier so as to drive the error signal to zero at the output of the error amplifier.
As is evident in FIG. 3, a full duplication of components is required for each frequency band. It can be appreciated that in a portable, hand-held device, such as a cellular telephone or a personal communicator, an important goal is to minimize the number of components. However, this has been difficult to accomplish with conventional directional couplers since each is typically tuned to the associated frequency band. As such, a directional coupler tuned for use at, by example, the 800 MHz band will not provide an accurate or usable output if operated at 1900 MHz. U.S. Pat. No. 5,363,071, issued Nov. 8, 1994 entitled "Apparatus And Method For Varying The Coupling Of A Radio Frequency Signal" by D. G. Schwent et al. describes a technique for increasing the dynamic range of a power detector in an automatic power control loop. The technique of Schwent et al. employs a plurality of switches that are coupled to a plurality of coupler elements. FIGS. 1-3 of this patent illustrate various embodiments of prior art RF couplers. The technique of Schwent et al. does not address the problem of providing a single RF coupler that can be used in two different frequency bands.